Hi all,
I'm new to this forum, but my topic isn't. I'm the new owner of the GrimReaper jet boat that's been discussed here before (http://datsun1200.com/modules/newbb/v ... 43&forum=1&post_id=133738, http://datsun1200.com/modules/mediawiki/index.php?title=Boats). And I still have the same issue - I need more power.

It's a tiny 2.9 meter jet boat that weighs maybe 500kg and is powered by an A12, which has just been re-built by the previous owner, but without any mods. With one person it goes fine, and under full throttle gets on the plane in a fairly short time. Once on the plane it goes pretty fast - speedo not working atm so not sure how fast, but quick enough to run around citycats. With 2 adults it can't quite get on the plane and so is painfully slow - tinnies passing us.

I talked to the mechanic who did the engine rebuild, and he seems to know what he's doing, and he suggested going turbo. One of the challenges is that the manifold is a DOEN marine manifold which also circulates the river water to cool the engine. Also, unless you pull the motor, you cant dyno a boat (easily). The proposal is to put on a Garrett T20 in a draw-through configuration to get to about 10psi boost, which he said the unmodded engine should be able to handle. BTW it has a 2 barrel weber, which he'll re-use.

It's all going to cost of few $$$ because I'm having him do all the work and there is a bit of fabrication involved due to the marine manifold and exhaust. So, question is, how much power boost could I expect with this? Any other ideas or things I should consider?
thanks

I'd go with a T25 off an S13 Silvia/180SX. There's heaps of them floating around. I've used one on an A12 before with very good results on 8psi. Pick up a cheap secondhand one, rebuild it with a carbon seal, drawthru setup plumbed back into the existing inlet manifold would be the way I'd go. Cheap, simple and effective. Blowthrough is far more trouble and expense than it's worth. Tuned properly, there's no reason it shouldn't be totally reliable and make easily double the stock power with good smooth driveability. Shouldn't have any dramas getting up on the plane! Personally, I'd change to a CD carb though. 1.5-1.75" SU or Stromberg would be better than a 2bbl carb in a drawthrough application.

Can you upload some pictures please? Especially pictures of the manifolds, inlet and exhaust.

I think a good draw through would give a good extra 20-40hp depending on how well you did the manifolds and timing. With access to E85 you could surely get a lot more HP.

Sounds like a fun boat.

thanks for the tip. Yeah, we were planning on drawthrough to keep things simple. At what revs would the T25 be effective? With the jet we'd need power pretty low and probably wouldn't rev pat 5500-6000 rpm.

here is a couple of different angles at the manifold and carby

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  A12 marine manifold.jpg (185.30 KB)


  A12 marine manifold 2.jpg (186.23 KB)

I remember it would make boost in 4th at 60km/h up steep hills. That works out to 2375rpm. Way more grunt right down at those rpm than non-turbo'd. And it made power to about 6500-7000rpm.

The only issue with turbo charging that I can see here with this application is going to be heat.
You are going to be retaining alot of heat from the turbo in a very confined space however after saying that I have seen jet skies with turbos so it can be done.
While what Harry is saying about a draw through is correct it seems a pity to me that with a ready supply of coolant ( river water ) it would be silly not to utilize this and run an water-air intercooler which means that you (in theory ) should be looking at a blow through system.
MG Metros had a blow through SU carb that come up for sale every now and again over here in NZ and would be ideal for this application.
You are going to have to fabricate new manifolds both inlet and exhaust as the watercooled marine manifold will not really work with a turbo application.
A simple heat exchanger can be easily fabricated up with small diameter copper tubing running river water coming from the jet unit contained within larger diameter tubing that is a sealed system containing coolant ( effectively a water cooled radiator )
Have you thought about fitting a supercharger instead ?Then you could run the manifold you are presently using.
Maybe not quite the same outright power of a turbo but I think that you would be pleasantly surprised and it definitely would pounce out of the water and up on the plane quicker.
Cheers
Dave

I don't see any reason he can't use the same inlet manifold he has now.

Also a jet prop will act like a torque convertor allowing slip relative to water speed. The thing will never be locked down to 2000rpm. No advantage in a superchanrger over a turbo IMO.

Many unlimited class ski boats used to run draw through twin turbo V8s with intercoolers before EFI became more popular. Never known any competitive boats to use blow through. But hey, there's also plenty of water around to use for water injection. And the manifold is water cooled...

I have not seen a a12 marine manifold but would imagine water is mixed before it exits the the manifold hence why it would need replacements.

the inlet manifold stays the same but the exhaust has some fabrication. Currently it has a mixer so water and exhaust exit through the transom. I'll get a separate exhaust made that will exit over top without mixing.

I'm looking at doing a second hand T25 now, instead of a new T20. With a draw through setup how important is it to get carbon seals? Or can you get a second hand T25 with carbon seals already in? Otherwise getting carbon seals put in I'm told means the tubo woudl have to be reworked and cost nearly the same as a new t20